Systems for displaying images (for both static and video images) have become a ubiquitous part of our everyday lives. For example, televisions provide viewers with news and entertainment. Display monitors used in computers and cellular phones enable users to interact with a variety of devices and various forms of information via images displayed on the monitors. High quality digital display systems have also emerged as possible replacements for physical media such as photographs and paintings. Recently, home theater systems with large projection display systems allow viewers to enjoy theater-like experience in their homes.
Despite the advancement of technologies relating to display systems, most conventional display systems only display two-dimensional (2D) images. A viewer, however, perceives the world in three dimensions by perceiving depth in addition to the horizontal and vertical dimensions. Because 2D images do not contain depth information, they appear to be less realistic to a viewer. A system that can display static or dynamic 3D images in high resolution is therefore desirable over 2D display systems. Moreover, in some situations, it is also desirable that a 3D display system simultaneously provide different perspectives of a 3D scene to viewers who are located at different angles with respect to the 3D scene. Such a system also allows a viewer to move around the 3D scene and gain different perspectives of the scene.
Several approaches have been used or proposed to display 3D images. One approach is to project two different images on one screen. The two different images contain the same scene captured from two different angles. A viewer is required to wears glasses that separate the combined image into the two different images. In particular, the glasses cause each eye of the viewer to perceive one of the two different images. This separation is possible because each image uses a distinct color (e.g., red and blue) or polarization. However, this approach suffers from the drawback that viewers must wear glasses in order to have a 3D experience.
A second conventional approach is to combine multiple 2D images of a scene, captured from different angles, into a single 2D image. In this approach, a set of adjacent pixels in the combined 2D image plays the role of a single pixel. Each pixel in the set of pixels corresponds to the same point in a scene, but has different brightness and color corresponding to a different perspective. A pinhole or a slit is placed at some distance from the set of pixels. For each point in the scene to be displayed, the pinhole or slit passes different color and brightness in different angles. Therefore, the eyes of a viewer perceive images that correspond to two different perspectives of a 3D scene. As a viewer moves around, the viewer also obtains different perspectives of the 3D scene being displayed. Instead of a pinhole or a slit, sometimes a lens is used. However, this approach suffers from the drawback that spatial resolution is significantly reduced.
A third conventional approach is to trade-off brightness resolution for generating the needed directional variation in displayed colors or brightness. In this approach, a screen is rotated at a very high speed. The rotating screen covers a 3D region of space. Each point in the 3D region is illuminated only when the screen passes through that point. The screen completes at least one full rotation during the time the eye integrates a single image. Therefore, the two eyes perceive images that correspond to two different perspectives of a 3D scene. In this case, an enormous amount of light energy is needed for the scene to appear crisp and bright. In addition, it requires the continuous mechanical movement of an entire projection system. As a result, it is difficult to scale such an approach to cover reasonably large display spaces. Finally, this approach is limited because it does not adequately handle points that are hidden from the viewer. Because one does not know a priori where the viewer is located, all points in the 3D scene are lit. Hence, the points that should be hidden can be seen “through” other visible points.
Yet another conventional approach is to use a display block formed by a set of liquid crystal sheets stacked together. The cells of the sheets are of the “scattering” type. A high frame rate projector is used to illuminate the stack of sheets where each projected frame is scattered by a single liquid crystal sheet while the remaining sheets are fully transparent and hence let light pass through to the viewer. Because the integration time of the eye is greater than the time it takes to illuminate all the sheets, the viewer perceives a volume that is lit up at the appropriate locations. This approach also suffers from the drawback that an enormous amount of light energy is required to create a bright 3D scene. In addition, points that should be hidden are always visible to the viewer.
In other conventional volumetric display systems, the display blocks are made of materials that can locally respond to specific types of illumination. In one example, fluorescent materials are used that glow when illuminated with laser beams shone from multiple directions. Such displays do not create a four-dimensional light field because the directional radiance of each point cannot be controlled. As a result, the displayed image is a collection of translucent (ghost-like) glowing points of light that are visible from all directions. Finally, holographic methods have been suggested several times in the past as a possible alternative. Unfortunately, conventional holographic displays can only display low quality images.